1. Field of the Invention
This invention relates generally to the field of measuring the volumetric flow rate of a fluid. More particularly, the invention relates to a method and system for measuring the volumetric flow rate of a fluid in a drilling rig return line.
2. Description of the Prior Art
Well control and blowout prevention have become important concerns in the oil and gas drilling industry for a number of reasons. Well blowouts cause higher drilling costs, possible loss of life, and waste of natural resources. An additional reason for concern is the increasing number of governmental regulations and restrictions being placed on offshore drilling operations partially as a result of recent, much-publicized well control incidents.
A kick can be defined as a well control problem in which the pressure found within the drilled formation is greater than the mud or fluid hydrostatic pressure acting on the borehole or face of the formation. This formation pressure causes fluids to flow from the formation into the well bore. In almost all drilling operations, the operator attempts to maintain a hydrostatic pressure greater than the formation pressure and thus prevent kicks. On occasion, however, and for various reasons, the formation pressure exceeds the mud pressure and a kick will occur. Kicks have become even more common due to the present trend of increasing drilling rates by using lighter drilling mud.
Another problem encountered when drilling a well is drilling fluid loss into the formation. This problem, known by the shorthand term, "Lost Circulation", occurs where the drilling fluid is flowing into a subterranean formation through which the borehole passes. Such condition should be detected quickly by a driller to prevent damage to such a formation and excessive loss of the drilling fluid.
A number of kick or lost circulation "indicators" can be observed at the surface before a kick has had time to result in a dangerous blowout or excessive time has elapsed since the beginning of lost circulation. Three of these are:
FLOW RATE CHANGE - An increase in the flow-out or flow rate leaving the well while pumping at a constant rate is one of the primary kick indicators. The increased flow rate is interpreted to mean that the formation is forcing formation fluids into the well bore. A decrease in the flow rate exiting from the well while pumping at a constant rate is an indicator of lost circulation.
FLOWING WELL WITH PUMPS OFF - When the rig pumps are not moving the mud, a continued flow-out from the well indicates that a kick is in progress. An exception to this indicator is when the mud in the drill pipe is considerably heavier than that in the annulus, as in the case of a slug.
PIT VOLUME CHANGE - If the volume of fluid in the pits is not changed as a result of surface controlled actions, an increase in pit volume indicates that a kick is occurring. The fluids entering the well bore as a result of the kick displace an equal volume of mud at the flow line and result in a pit gain. A decrease in pit volume under these conditions indicates lost circulation.
Two of the kick early warning signs described above require measurement of an increase in flow rate from the fluid return line, while the other requires measurement of an increase in pit volume. These indicators are difficult to interpret when drilling from a floating drilling vessel because of the heaving and rolling of the drilling vessel in response to wind and waves. Floating drilling vessel heaving and rolling creates fluid return line flow rate changes.
It has been found that the time elapsed between the beginning of a kick deep in the well and its detection at the surface by pit level monitoring is too long to provide sufficient time to bring the well under control such as by adding weight to the drilling fluid.
Studies have shown that accurate differential flow measurements, of the order of twenty-five gallons per minute (25 GPM) provides the earliest possible surface detection of kicks and/or lost circulation. Such high absolute accuracy under widely varying conditions for both flow-in and flow-out systems, however, is difficult to obtain with the systems of the prior art.
Presently, flow-in measurement is based on the number of strokes per minute of triplex mud pumps (see FIG. 1). The flow rate obtained from the pump strokes is then corrected by a volumetric pump efficiency. This pump efficiency can fluctuate between 80% to 95% accounting for inaccuracies of plus or minus seven and one half percent in the flow-in measurement.
The prior flow-out measurement has usually included a "paddle" system installed in the rig return line. The paddle is a hybrid flow meter based on level and target (force) measurements. The prior art paddle has an uncalibrated accuracy of around forty percent. With calibration on the rig site, the "absolute" flow-out measurement is still only accurate to ten or fifteen percent due to the basic non-linearity of the device, and due to very poor zero stability of the device. Poor zero stability requires frequent recalibration.
Using the pump strokes and the paddle measurement for flowing and flow-out respectively, the best accuracy for the differential flow over the entire fluid flow range cannot be much better than about twenty-five percent, or three hundred (GPM) in twelve hundred GPM. This is more than ten times the required accuracy, rendering prior methods of differential flow rate measurement inadequate for desired kick detection.
Electromagnetic flow meters have also been used but have drawbacks. They do not work in oil based muds (conductivity too low). They require complete modification of the return line. In offshore rigs where modification of the return line is difficult and space is limited, there is usually no way to install them. They require expensive maintenance to sustain their accuracy.
Other industries have developed flow measuring systems suitable for use in waste water monitoring systems where sewer outflows must be monitored for pollution control purposes. These systems obtain flow measurements based on the velocity of the fluid in a channel and the area of the channel occupied by the flowing liquid. Ultrasonic level detectors and Doppler type velocity detection units have been used for these applications.
U.S. Pat. No. 4,217,777 to Newman issued Aug. 19, 1980 discloses such a system and is incorporated herein for essential material and for all other purposes. Also U.S. Pat. No. 4,202,211 to Perry issued May 13, 1980 discloses a similar system and is incorporated herein for essential material and for all other purposes.
Ultrasonic level detection systems are known in the art. Such systems are described in U.S. Pat. No. 4,024,766 to Perry issued May 24, 1977, U.S. Pat. No. 4,145,914 to Newman issued Mar. 27, 1979, and U.S. Pat. No. 4,228,530 issued Oct. 14, 1980, all of which are incorporated herein for all purposes.
Further, U.S. Pat. No. 4,754,641 to Orban et al., while providing improved results relative to the other methods for measuring fluid flow in return lines, still suffers from inaccuracies due to the requirement of a velocity probe which is inherently inaccurate in measuring mud flow in a drilling rig return line due to the wide range of elements in the mud. Thus, even with this advance, the art does not provide a method for sufficiently accurately determining a volumetric fluid flow rate such that a kick or lost circulation determination can be made in real time on a drilling rig.